1. Technical Field
The present invention generally relates to antenna systems and, more particularly, to a cavity-backed periodic slot array antenna with a compact conformal and low profile structure that realizes wide frequency bandwidth.
2. Discussion
Low profile conformal antennas have become particularly useful for transmit and receive communications systems such as advanced identification of friend or foe (AIFF), data link and satellite communications systems. These and other types of communications systems are often selected to operate over various selected frequencies and the useful frequency range is generally dependent on the antenna design. In this regard, conventional slot antennas and printed microstrips for patch antennas have been developed and used for such applications and can generally be made with small low profile structures.
In particular, cavity-backed slot antennas have been mounted on the outer surface of aircraft and on various other airborne and ground objects in the past. The conventional slot antenna typically included a slot etched in a conductive surface near a conductive cavity which in turn communicates with a feed line. The physical dimensions of the slot and conductive cavity generally determine the effective frequency range of operation. For instance, for phased array applications, the spacing between elements of the array should generally be kept less than one-half the wavelength of the operating signals to avoid any potential grating lobes in the antenna pattern. However, many of the conventional cavity-backed slot antennas are generally effectively limited to a narrow frequency bandwidth. Hence, in order to achieve a cavity-backed slot antenna that is more useful for applications which require a wider frequency range, the frequency bandwidth needs to be extended.
More recently, cavity-backed log-periodic slot array antennas have been developed and are generally made up of a plurality of individual slot and cavity elements configured in a linear array according to a log-periodic scale. That is, the physical dimensions of width, length and thickness of the individual cavities generally increase from small to larger cavities in accordance with a log-periodic scale. In addition, the radiating slots typically extend along a portion of the width of the corresponding cavity and likewise increase in size with larger width cavities. By employing several varying size cavities and slots, the overall frequency bandwidth of the antenna can be extended over a wider frequency range.
The log-periodic slot array advantageously performs more like a frequency independent antenna with uniform gain and end-fire pattern shape over an extended frequency range. However, the conventional log-periodic slot array generally suffers from a number of deficiencies. For example, the conventional log-periodic slot array can become quite large, especially in width, and therefore often impracticable for use in many airborne phased array applications. This is generally due in part to the fact that the conventional slots extend the width of the individual conductive cavities and the conductive cavities are formed with an inside physical dimension to accommodate lower frequency signals. Also, the non-constant thickness of the conductive cavities of the conventional slot arrays is difficult to fabricate and can be excessive in size, especially for large arrays, making it difficult to realize a low profile conformal configuration. Additionally, beamwidth of the conventional log-periodic slot array can be too narrow for use in a wide angle scan phased array. Also, cross-polarization levels may be high at wide angles with many known conventional approaches.
It has become increasingly important that antennas employed for avionics systems and the like, such as those used on high performance fighter aircraft, be conformal and have a very low profile structure suitable for use on the exterior surface of the aircraft. However, the conventional conformal and low profile antennas, in general, have inherent characteristics that often make it very difficult and sometimes impracticable to employ a conventional slot antenna that meets a particular set of stringent performance requirements which are often imposed for such aircraft use and the like.
It is therefore desirable to provide for a compact log-periodic slot array antenna that is suitable for conformal low profile wideband phased array applications. It is also desirable to provide for such a compact slot array antenna which may provide forward horizon coverage and be capable of providing wide angle beam scanning. It is further desirable to provide for such a slot array antenna that may be used on advanced avionic systems such as high performance fighter aircraft and offers a low profile and conformal structure. Yet, it is also desirable to provide for a cavity backed slot antenna element with small physical dimensions and which is able to realize an extended frequency bandwidth.